Materials and plasma processes continue to put demands on excitation methods. Recently, the high frequency excitation that is in the microwave region 300 MHz to 1,000 GHz has demonstrated the production of unique conditions for materials and plasma processes. For example, plasmas excited by microwaves can exist under conditions not easily achieved by other methods from about 10.sup.-3 Torr to greater than atmospheric pressure. Coupled with magnetic fields, an Electron Cyclotron Resonance (ECR) region can be achieved producing intense plasmas in the range of from 10.sup.-2 to 10.sup.-7 Torr
Microwave power is very useful for generating plasmas for several reasons. It is a very efficient source of power for the free electrons. The time average power absorbed by these electrons is proportional to the electron density and the effective electric field squared. The effective electric field has a maximum value when the frequency of the power source equals the effective electron collision frequency. (Refs. 1 and 2) For gases typically used in plasma processing this electron collision frequency is in the microwave region of the spectrum. Most current plasma processing techniques (etching and deposition) currently use a frequency of 13.56 MHz, e.g., Diode sputtering, Reactive Ion Etching, etc.. For microwave radiation the frequency is a few hundred to several thousand times this. The frequency commonly used in microwave plasma processing is 2.45 GHz. 2.45 GHz is one of the designated Industrial Scientific Medical (ISM) oven heating frequencies. This is the frequency that is used in most commercially available home microwave ovens. Power supplies that operate at this frequency are readily available. They make use of a microwave tube called a magnetron to generate microwave energy. For a given power input, microwaves result in a greater electron density in the plasma than RF plasmas due to the frequency effects. Since the plasmas are almost neutral, this results in a higher ion density. This higher ion density results in higher processing rates, i.e., higher deposition and etching rates.
Another advantage of microwave processing is related to how the power is coupled to the plasma. Although there are many different ways of coupling microwave power to a plasma, (Ref. 3 and the references cited therein) they have one useful feature in common. The metal electrodes of the applicator are usually external to the vacuum system containing the plasma. The microwave power is coupled through a dielectric medium such as fused quartz or sapphire. This results in less contamination of the plasma and the parts being processed by the metal electrodes usually used with lower frequency processes and with reactive plasmas such as oxygen, chlorine, fluorine and the like. In addition to the decreased contamination, there is the elimination of the consumption of the metal electrodes by these reactive species.
Due to higher electron densities, microwave driven plasmas are more efficient sources of radicals that are used in plasma processing. By controlling the plasma conditions, particular species of radical production may be enhanced. In general, high pressure (&gt;10 mt) microwave discharges are good sources (i.e., high density) of radical species. (Ref. 4) Furthermore, in a microwave driven plasma, the temperature (energy) of the neutral gas component of the plasma can be much higher than in the more conventional lower frequency plasmas. Mechanisms, such as viscous heating of the neutral species by the energetic electrons, can result in higher processing reaction rates.
In addition to the denser plasmas that can be o obtained with the higher frequency microwave power, there are resonant phenomena that occur that further enhance the coupling of microwave power to the plasma. Electron cyclotron resonance or ECR as it is called is an example of this enhanced coupling. For ECR the frequency of electron motion around a magnetic field direction (determined by the magnitude and direction of the magnetic field and the electron velocity) is the same as the frequency of the microwave radiation. For 2.45 GHz radiation a magnetic field with a magnitude of 875 Oersteds is needed. When this condition is met, there is an enhancement in the power absorbed by the free electrons of the plasma. With ECR enhancement greater than 10%, ionization is possible (Refs. 5 and 6).
In semiconductor and other plasma processing applications, intense microwave discharges are important in many applications such as deposition, etching, ashing, and ion beam generation. Many commercial systems are available to do microwave processing. Testing has shown that microwave technology has considerable advantage over existing technology. However, most existing microwave processing technology is limited in that it relies on vacuum tube technology, therefore, the limitations are imposed on the processes. Further, most plasma and material processing supplies are based on commercial home oven magnetrons operating at 2.45 GHz and are therefore limited in the applications they can be used in, due to the inherent limitations of tube technology.
Microwave solid material processing per se is becoming increasingly important as a manufacturing processing technique. These processes include heating, curing, sintering, annealing or, in general, any process that directly couples microwaves into a solid or liquid material intending to change the chemical or physical structure of that material. Microwave excitation has several advantages over conventional techniques such as thermal or chemical processing. The microwave process is usually more efficient and faster. End point detection of the microwave process is usually available by monitoring, for example, the forward and reflected power (See for example Ref. 7). These advantages result from the direct coupling of the microwave energy into the chemical bond (usually a dipolar interaction). Because the energy is coupled internal to the system, the processing takes place at a faster rate than in conventional processing. For example, microwave curing of polymers results in lower thermal stress within the material than other techniques due to the direct energy coupling which reduces processing time. Further, as the material cures, it absorbs less energy except in regions still curing.
Microwave processing systems are available commercially. These systems have significant disadvantages that limit their use in some critical applications, again, through the use of oven magnetrons. Lack of microwave electromagnetic field uniformity is one disadvantage. Although some of these systems use resonant applicators or slotted waveguides, most of these systems are little more than multi-moded microwave ovens. In these systems, for regions of low electromagnetic field, the curing will be slower than in regions of high electromagnetic fields. Another important problem that currently available tube type microwave systems have is a lack of process control. In the typical oven type system it is very difficult to tell when a process is completed and to correct for problems in the process, such as overheating.
Two different types of microwave sources, solid state and vacuum tube, are currently known in the art. There are many different types of tube sources of microwave power, e.g., magnetrons, klystrons, gyrotrons, and the like (Ref. 8). Tubes have advantages in that they are capable of operating at higher frequencies and higher power levels. Power output and to a limited extent, frequency, can be adjusted with tube type sources. This can be done with dc bias voltages and magnetic fields. However, with the magnetron source, the most readily available and least expensive of the tube-type sources (for low power levels--less than several kW), the phase cannot be controlled. The other types of tube sources can be used with phase control, but they are very expensive and operate at the higher power levels of several tens of kW and more. Their main disadvantages are that they are large, bulky, very expensive, heavy and require high voltage from the anode to the cathode and high current for a filament. Tubes are also difficult to control and have short lifetimes.
In general all microwave tubes produce microwave power by converting the kinetic energy of an electron beam in a vacuum into electromagnetic energy. Different types of tubes employ different coupling structures to do this. No coupling structure is optimal, each has its advantages and disadvantages.
For example, traveling wave tubes use a helix coupling structure to convert the energy of the electron beam into electromagnetic energy. A klystron uses a series of cavity couplers to do the same thing. A magnetron bends the electron beam into a helix using a magnetic field, then converts the kinetic energy of the electron beam into microwave energy using a series of tuned cavities mounted radially around the center axis.
All of these tubes have similar advantages and disadvantages associated with their use when compared to solid state devices. All of these tubes require a source of electrons. In all microwave tubes, this is provided by a hot filament. To drive the filament, a low voltage high current supply is required. The filament causes problem in that it is very sensitive to vibration, produces large energy losses due to its inefficient nature, contributes to the heating of the tube and limits the lifetime of the tube. In general, filament failure is one of the leading causes of tube failure.
The second requirement of microwave tubes is an electron beam moving at a considerable velocity. This places more requirements on the design and use of the tubes. To accelerate the beam and keep it from spreading, a high potential is required in a high vacuum environment. This required potential, in turn, requires the use of a high voltage, low current supply. These supplies are expensive, bulky and difficult to operate. To prevent scattering of the beam, the tube is designed as a high vacuum chamber. This contributes greatly to the cost and complexity of the microwave tube. Most tubes have getters designed into the filament structure to maintain this vacuum. However, loss of vacuum integrity is the second most common failure mode in microwave tubes.
The third requirement in a microwave tube is a coupling structure to convert the energy in the electron beam into microwave power. As mentioned earlier, these structures vary in different tubes. However, all tubes share common problems with its use. These structures are very complex and difficult to machine. Typical machine tolerance is one ten thousands of an inch. Also microwave absorption in these structures is a big source of heat in these tubes. This requires some method of cooling that adds to the cost, bulk and complexity of the tube and the overall supporting structure.
A noteworthy example of volume production economics is the home microwave oven operating at 2.45 GHz. Because of the millions of tubes produced every year, manufacturing costs have been reduced to about five dollars per tube. These problems have been somewhat alleviated by spreading development and tooling costs over the large number of tubes produced. Through iterative improvements, problems such as cooling, lifetime and size have been addressed. Although, these tubes are reasonably compact and have reasonable lifetime (e.g. 2000 hours or more), their power supplies are still bulky and require high voltage but are easy to obtain. However, the tubes are oscillators and thus operate only at 2.45 GHz. Further, since they are in effect a "diode" device, they oscillate effectively at full power. This means that there is no effective way to control the frequency or power output of these tubes.
It should be noted, in general, that a microwave tube can be built that can match the overall performance characteristics of any solid state device such as bandwidth and output power. However, it will cost approximately a thousand times more, be many times larger and heavier, consume more power, require complicated supply voltages, have limited lifetimes and have minimal controls of power output and other operating parameters such as phase, bandwidth and noise.
Except for military radar and communication applications, all microwave power generators in use currently are based on microwave tube technology. These tubes were developed during World War II and have changed very little functionally since then. These tubes can generate large amounts of power at frequencies up to the tera Hertz range. However, as stated previously, these tubes are heavy, bulky, inefficient, require high voltages and currents, expensive and have short lifetimes. All of these factors are very important in a manufacturing environment.
Because of all of the aforementioned problems with microwave tubes, most existing commercially available microwave supplies use oven magnetrons operating at 2.45 GHz. This limits the equipment and the applications it can be used in.